The transmission mode photocathode is desired to perform detection with linearity in a wide range of small to large light quantities, or, to improve its cathode linearity characteristic. The cathode linearity characteristic herein means linearity of cathode output current against incident light quantity. For improving the cathode linearity characteristic, it is necessary to implement appropriate charge supply to a photoelectric conversion layer and it can be considered that the necessity is met, for example, by placing an electroconductive layer (underlying layer) between an optically transparent substrate and the photoelectric conversion layer to reduce the surface resistance of the photoelectric conversion layer.
On the other hand, for a reflection photocathode, there is a known configuration wherein a layer of graphite and carbon nanotube or the like (intermediate layer) is placed between a substrate and a photoelectric surface (cf. Patent Literature 1 below).